With regard to the first of the potential uses mentioned above, it is possible to convert wind energy to electrical energy by using a wind turbine to drive the rotor of a generator, either directly or by means of a gearbox. The ac frequency that is developed at the stator terminals of the generator (the “stator voltage”) is directly proportional to the speed of rotation of the rotor. The voltage at the generator terminals also varies as a function of speed and, depending on the particular type of generator, on the flux level. For optimum energy capture, the speed of rotation of the output shaft of the wind turbine will vary according to the speed of the wind driving the turbine blades. To limit the energy capture at high wind speeds, the speed of rotation of the output shaft is controlled by altering the pitch of the turbine blades. Connection of the variable voltage and frequency of the generator to the nominally fixed voltage and frequency of the supply network can be achieved by using a power converter.
The power converter typically includes a generator bridge, which in normal operation operates as an active rectifier to supply power to a dc link. The generator bridge can have any suitable topology with a series of semiconductor power switching devices fully controlled and regulated using a pulse width modulation (PWM) strategy.
The dc output voltage of the generator bridge is fed to the dc terminals of a network bridge, which in normal operation operates as an active inverter. The principal control for the dc output voltage is achieved by controlling the generator bridge but other methods of controlling the de link voltage are possible. The network bridge can have any suitable topology with a series of semiconductor power switching devices fully controlled and regulated using a PWM strategy.
The ac output voltage of the network bridge is filtered and supplied to the nominally fixed frequency supply network via a step-up transformer. Protective switchgear can be included to provide a reliable connection to the supply network and to isolate the generator and converter system from the supply network for various operational and non-operational requirements.
The power that is exported to the supply network must meet the requirements defined in the various standards and grid codes. For example, in one of the relevant standards, the amplitude of the harmonic voltage distortion relating to the sidebands of the switching frequency should be kept below 0.2% of the voltage amplitude of the voltage waveform of the supply network at the fundamental frequency.
The PWM strategy used in the network bridge will typically operate at a given switching frequency. The mixing between the nominally fixed frequency of the power grid or supply network and the switching frequency of the PWM strategy will cause harmonics in the ac output voltage of the network bridge. If two or more power converters are connected to a common supply network or power grid (for example, in the case of a wind turbine farm where a plurality of wind turbines might be connected to a supply network through a parallel connection) then the overall harmonic voltage distortion in the power that is exported to the supply network may exceed the required limits defined for the common point.